United States Patent [191 Keir

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5,467,400 Nov. 14, 1995

Patent Number:

Date of Patent:

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[45]

[54] SOLID STATE AUDIO AMPLIFIER

[75]

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EMULATING A TUBE AUDIO AMPLIFIER

Inventor: Kingdom

Assignee: Keynes, United Kingdom

Appl. No.:

PCT Filed:

PCT No.:

§ 371 Date:

87,767

Jan. 14, 1992

PCT/GB92/00075

Jan. 18, 1994

§ 102(6) Date: Jan. 18, 1994

PCT Pub. No.: WO92J13388

PCT Pub. Date: Aug. 6, 1992

Foreign Application Priority Data

Bruce Keir, Milton Keynes, United

Marshall Ampli?cation Plc, Milton

Jan. 17, 1991 [GB] United Kingdom ................. .. 9101038

Int. Cl.6 ..................................................... .. H03G 3/00

US. Cl. ............................................... .. 381/61; 381/98

Field of Search ........................................ .. 381/61, 98

[51] [52] [58]

[56] References Cited

FOREIGN PATENT DOCUMENTS

0050067 4/1982 0181608 5/1986 2068680 8/1981 2103004 2/1983

European Pat. O?". . European Pat. Oil". .

OTHER PUBLICATIONS

Electronic Engineering v. 50 No. 608, Jun. 1978, pp. 17-19 J. E. Morris “Active Filter Using Norton Op-Amp Gyra tors”.

Primary Examiner-Stephen Brinich Attorney, Agent, or F irm—Hodgson, Russ, Andrews, Woods & Goodyear

[57] ABSTRACT

A solid state audio ampli?er for connection with a loud speaker cabinet or emulation thereof to provide ampli?ca tion characteristics emulating in frequency response those of a valve audio ampli?er driving a loudspeaker cabinet. The loudspeaker cabinet comprises a loudspeaker drive and its enclosure. The frequency response of the audio ampli?er exhibits a low frequency resonant peak in gain representing the fundamental resonance of the loudspeaker cabinet and a monotonic increase in gain at higher frequency associated with the impedance provided voice coil inductance of the loudspeaker cabinet. The audio ampli?er comprises an ampli?er and gain control circuitry associated therewith. The ampli?er comprises a differential ampli?er and the gain control circuitry comprises a impedance network connected in feedback. The impedance network comprises a tuned circuit connected to create the low frequency resonant peak in gain and a reactance providing the increase in gain at higher frequency. The tuned circuit comprises a capacitance and a gyrator circuit simulating an inductance and the reactance comprises a capacitance.

United Kingdom .

United Kingdom . 6 Claims, 3 Drawing Sheets

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SOLID STATE AUDIO AMPLIFIER EMULATING A TUBE AUDIO AMPLIFIER

BACKGROUND OF THE INVENTION

This invention relates to audio ampli?ers. It is well accepted that with electrically ampli?ed guitar

and similar music, the characteristics of the ampli?cation circuitry and loudspeakers play a fundamental role in attain ing the desired sound. Since the l960’s, valve-based guitar ampli?ers driving traditionally constructed guitar-type loud speaker cabinets, have become associated with a character istic and highly regarded sound quality. These electrical valve ampli?ers remain in high demand in particular sectors of the music industry, despite the fact that valve technology is regarded as obsolete in many other electrical ?elds.

Numerous attempts have been made to emulate, using solid-state circuitry, the characteristic sound of a valve ampli?er and traditional loudspeaker cabinet. Efforts have been made to produce line level audio signals which when reproduced through headphones, recording equipment or full range linear loudspeakers, simulate the sound achieved with a traditional loudspeaker cabinet. Thus, solid state guitar ampli?ers are available with distortion elfects delib erately introduced to imitate the distortion inherent in valve ampli?ers. Loudspeaker emulaters have been proposed (see for example WO 88/00410) which provide a valve ampli?er with a reactive load impedance equivalent to that of a traditional loudspeaker cabinet and which ?lter the audio signal in simulation of the frequency response of a loud speaker.

This invention is directed to solid state ampli?ers which can be used either to drive traditional loudspeaker cabinets or, with appropriate loudspeaker emulation, to provide line level signals. It is an object of this invention to provide an improved solid-state ampli?er or ampli?er stage which reproduces more faithfully the output of a valve ampli?er driving a traditional loudspeaker cabinet.

It has been found by the present inventor that when driving either a traditional loudspeaker cabinet or a loud speaker emulator, it is not enough for a solid state ampli?er to simulate the distortion and related characteristics of a valve ampli?er. It is now recognised that the complex load impedance provided by a traditional loudspeaker cabinet has-by virtue of the comparable output impedance of the valve ampli?er-a marked elfect upon the effective gain of the ampli?er. If the valve amplifer is replaced by a solid state ampli?er which has the identical “bench” characteristics but a low output impedance, this frequency-dependent gain effect is lost. There is accordingly a degradation in sound quality irrespective of whether the ampli?er is used to drive a traditional loudspeaker cabinet or an appropriate emula tron.

An approach to this problem is to provide a solid state ampli?er with an arti?cially high output impedance. There are, however, signi?cant problems with this approach. For example, a resistive load connected in series with the ampli?er output will dissipate unacceptable high power levels. Alternatives exist but it is still desirable to retain the low output impedance normally associated with solid state technology.

SUMMARY OF THE INVENTION

It is accordingly a,n object of this invention to provide an improved audio ampli?er stage which provides, without

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2 high output impedance, an audio signal simulating in the respects in question the output of a valve ampli?er when driving a traditional loudspeaker cabinet. That audio signal can optionally be used with a traditional loudspeaker cabinet or appropriate loudspeaker emulation.

Accordingly the present invention consists in one aspect in solid state audio ampli?er means for connection with a loudspeaker cabinet or emulation thereof to provide ampli ?cation characteristics emulating in frequency response those of a valve audio ampli?er driving a loudspeaker cabinet, wherein the frequency response of the audio ampli ?er means is adapted to exhibit a low frequency resonant peak in gain representing the fundamental resonance of the loudspeaker cabinet and a monotonic increase in gain at higher frequency associated with the voice coil inductance of the loudspeaker cabinet.

Preferably, the audio ampli?er means comprises an ampli ?er and gain control circuitry associated therewith.

Advantageously, said ampli?er comprises a differential ampli?er and said gain control circuitry comprises a imped ance network connected in feedback.

Preferably, said impedance network comprises a tuned circuit connected as to create said low frequency resonant peak in gain and a reactance providing said increase in gain at higher frequency.

Suitably, said tuned circuit comprises a capacitance and a gyrator circuit simulating an inductance and said reactance comprises a capacitance. The present invention will now be described by way of

example with reference to the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plot of impedance versus frequency for a typical loudspeaker cabinet of traditional construction;

FIG. 2 is a plot of gain versus frequency for a typical valve ampli?er driving the loudspeaker cabinet of FIG. 1;

FIG. 3 is a circuit diagram of an ampli?er stage according to the present invention;

FIG. 4 is a plot of gain versus frequency showing the response of the circuit shown in FIG. 3; and

FIGS. 5 and 6 are circuit diagrams of ampli?er stages according to further embodiments of this invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Whilst a loudspeaker has a nominal impedance rating of—say—8 ohms, this value generally only applies over a small, mid-frequency range. At higher and lower frequencies there are considerable variations in impedance which are characteristic of the loudspeaker and its enclosure. There is shown in FIG. 1, a plot of impedance versus frequency for a MARSHALL (Registered Trade Mark) 4X12 loudspeaker cabinet. The plot is logarithmic and it will be seen that there is a peak in impedance at low frequency. This is due to the fundamental resonance of the loudspeaker drive unit and its enclosure. At frequencies above 400~500 HZ the impedance continues to rise with frequency, because of the inductance ‘ associated with the loudspeaker voice coil.

It will be apparent that the voltage developed at the output terminals of a power ampli?er will depend upon the output impedance of the ampli?er and the impedance of the loud speaker, as illustrated for example in FIG. 1. If the output

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impedance of the ampli?er is small, then changes in the value of the loudspeaker impedance will have only a small e?fect on the output voltage. Conversely, if the output impedance of the ampli?er is comparable with the loud— speaker impedance, changes in the value of the output impedance will then have a signi?cant effect on the output voltage of the ampli?er. This latter situation applies, as has been explained, to a valve ampli?er, with the eifect that the response of the ampli?er shows a marked frequency depen dence. There is shown in FIG. 2, by way of illustration, a plot of the gain of a 100 watt MARSHALL (Registered Trade Mark) power ampli?er driving a loudspeaker cabinet as mentioned above. If a solid-state ampli?er of low output impedance were to be used to drive the identical loudspeaker cabinet, the sound would be degraded through loss of this characteristic frequency response.

There is shown in FIG. 3 an ampli?er stage according to this invention which provides a good approximation to this characteristic frequency response. The circuit is based on a differential ampli?er ICla used

in non-inverting mode. An input signal is applied via capaci tor C1 to remove any DC components, with resistor R1 providing a bias path to ground.

The gain of the ampli?er ICla is controlled in feedback. For the purposes of explanation, the components may be grouped, as shown in the ?gure in dotted outline, in complex impedances Z1 and Z2. It will then be understood that the gain of the ampli?er is given by:

v0 (1)

The eifective impedance Z1 comprises principally resistor R3. Capacitor C2 is provided in parallel with the resistor R3 for a purpose which is not germane to the present invention. In conventional manner, capacitor C2 provides for a roll-off in the gain of the ampli?er above the frequency range of interest.

Effective inductance Z2 comprises two parallel connected paths. One path comprises capacitor C3 and resistor R2 connected in series between the inverting input of the ampli?er ICla and ground. The other path comprises the series connection of capacitor C4 and a gyrator circuit serving the function of an inductance.

More speci?cally, the gyrator circuit comprises ,resistors R4 and R5 connected in series between capacitor C4 and ground. A differential ampli?er IClb has its non-inverting input connected to the junction of capacitor C4 and resistor R4. The inverting input is connected in feedback to the output, which is connected in turn through capacitor C5 with the junction of resistor R4 and R5. In a manner which is known per se, this circuit simulates an inductor. At low frequencies, capacitor C5 is essentially open-circuit so that the impedance of the gyrator circuit is detemiined by resistors R4 and R5. At higher frequencies, the function of the differential ampli?er will be to apply at the junction of resistor R4 and R5, essentially the same voltage appearing at the junction of capacitor C4 and resistor R4. Thus at high frequencies, there is no voltage drop across resistor R4, no current ?ows and the eifective impedance is high, as with an inductor.

The combination of capacitor 4 with the described gyrator circuit provides a series tuned circuit having, in this speci?c example, a centre frequency of approximately 125 HZ and a Q factor of 2.3. It will be recognised that the impedance of the series tuned circuit falls to a minimum at the resonant

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4 frequency. The frequency dependence of the combined inductance

Z2 can now be understood. At low frequency, capacitor C3 is eifectively open~circuit so that the series tuned circuit is dominant. As explained, this impedance shows a resonant drop at approximately 125 HZ. As will be clear from the equation (1), the effect of a drop in Z2 is to increase the gain of the ampli?er stage. Thus a low frequency peak in ampli ?er gain is produced, as desired. At high frequencies, the path containing capacitor C3 and

resistor R2 will dominate, producing a capacitative drop in impedance with frequency. A decrease in Z2 produces an increase in the gain of the ampli?er, so that a steady increase in gain at higher frequencies is provided, again as desired.

These qualitative phenomenon are illustrated quantita‘ tively in FIG. 4 which plots the frequency response of the circuit shown in FIG. 3. A comparison between FIGS. 2 and 4 will show that the ampli?er stage according to the inven— tion has had the desired effect of emulating the frequency response of a high output impedance valve ampli?er con nected with a traditional loudspeaker cabinet.

The output of the ampli?er stage shown in FIG. 3 will typically be used with appropriate loudspeaker emulation to drive headphones, recording equipment or other linear trans ducers. At appropriate power levels, however, the ampli?er stage according to this invention can be used to drive conventional loudspeaker cabinets. The described circuit is of course only one example of the

manner in which the gain of the ampli?er can be controlled so as to produce the desired frequency of response. In a simple modi?cation, the described gyrator circuit can be replaced by a true inductance although this is likely to be less compact and more expensive. In a more radical modi ?cation, the circuit can be arranged so that gain is controlled through the combined impedance Z1. Thus, for example, in the circuit of FIG. 5, Z1 comprises two parallel arms. A ?rst arm comprises the series connection of an inductor L1 with a parallel tuned circuit of inductor L2 and capacitor C2. The second arm comprises resistor R1. Inductance Z2 is formed by resistor R2. At low frequencies, the impedance of induc tor L1 is low so that the behaviour of combined impedance Z1 is dominated by the parallel tuned circuit. This is selected to have a resonance at a low frequency (typically 125 HZ). The impedance of a parallel tuned circuit is at a maximum at resonance, giving the required resonant peak in gain at low frequency. At higher frequencies, the inductor L1 will dominate, providing a steady increase in the value of imped ance Z1 and thus a steady increase in gain, according to the desired characteristic. Resistor R1 serves to prevent gain moving to in?nity in the case of lossless inductors. The skilled man will appreciate that it will be possible to

drive a di?‘erential ampli?er in the inverting mode, with appropriate feedback control to produce the desired gain characteristic.

Reference is directed, for example, to FIG. 6 in which the input signal Vin is taken through an impedance network Z1 to the inverting input of a differential ampli?er ICla. The non-inverting input is taken to ground and negative feedback is applied through impedance network 22 in the form of resistor R2. In this arrangement it will be understood that the gain of the ampli?er stage is given by

V0 (2) Vin

The impedance network Z1 comprises three parallel arms,

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one consisting of a series tuned circuit C1, L1 with series resistor R4, the second a resistor R1 in series with capacitor C2, and the third a resistor R3.

The behaviour of impedance Z1 determines the frequency response. At low frequency, C2 is effectively open circuit and the resonant drop in impedance of the series tuned circuit C1, L1 provides the desired resonant peak in gain. At higher frequencies, the diminishing impedance of C2 pro vides the desired steady increase in gain. Resistors R3 and R4 serve to keep the gain within range, in the case of lossless inductors and capacitors.

It will be recognised that, by analogy with FIG. 5, the' gain of an ampli?er driven in non-inverting mode can similarly be controlled through impedance Z2. Thus, a parallel tuned circuit can provide the necessary resonant increase in imped ance, with an inductance producing a steady increase in impedance with frequency. The inductance may of course be in the form of a gyrator circuit.

Generally, it is more convenient to employ capacitors rather than inductors and to use gyrator circuits simulating inductance, rather than true inductors.

Still further alternatives will occur to the skilled man in this art, by which the gain of the ampli?er can be controlled to produce the desired resonant increase in gain at low frequency associated with the fundamental resonance of the loudspeaker cabinet, and the monotonic increase in gain at higher frequencies associated with the voice coil inductance of the loudspeaker. Although, it will often be more conve nient to employ feedback or other gain control circuitry to give the desired ampli?er frequency response, the altema tive exists of combining an ampli?er with circuitry provid ing controlled attenuation over speci?ed frequency bands.

I claim: 1. Solid state audio ampli?er means for connection with

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6 a loudspeaker cabinet or emulation thereof to provide ampli ?cation characteristics emulating in frequency response those of a valve audio ampli?er driving a loudspeaker cabinet, said loudspeaker cabinet comprising a loudspeaker drive and the enclosure thereof, so that when said solid state audio ampli?er means is connected to said loudspeaker drive the frequency response of the audio ampli?er means exhibits a low frequency resonant peak in gain representing the fundamental resonance of the loudspeaker cabinet compris ing said loudspeaker drive and said enclosure and a mono tonic increase in gain at higher frequency associated with the impedance provided by voice coil inductance of the loud speaker cabinet comprising said loudspeaker drive and said enclosure.

2. Ampli?er means according to claim 1, comprising an ampli?er and gain control circuitry associated therewith.

3. Ampli?er means according to claim 2, wherein said ampli?er comprises a differential ampli?er and said gain control circuitry comprises a impedance network connected in feedback.

4. Ampli?er means according to claim 3, wherein said impedance network comprises a tuned circuit connected as to create said low frequency resonant peak in gain and a reactance providing said increase in gain at higher fre quency.

5. Ampli?er means according to claim 4, wherein said tuned circuit comprise: a capacitance and a gyrator circuit simulating an inductance and said reactance comprises a capacitance.

6. Audio ampli?er means according to claim 1 in com bination with the loudspeaker cabinet comprising said loud speaker drive and said enclosure, wherein said audio ampli ?er means is connected to said loudspeaker drive.

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